In all kinds of optical disk systems, such as compact disk players or DVD players, optical heads irradiating laser light that is focused by an object lens onto a recording plane are used for reading information from recording media and writing information onto recording media. To increase the recording density of the recording medium, it is necessary to make the focus spot diameter of the object lens smaller, and therefore it is preferable that the image-side NA (numerical aperture) of the object lens is large. Furthermore, striving for a reduction of the reproduction and access times, it is important that the object lens is small and light.
Conventionally, aspherical plastic lenses with NA.apprxeq.0.45 were used for the object lens in compact disk players, and aspherical molded glass lenses with NA.apprxeq.0.60 were used for the object lens in DVD players. In any case, conventional object lenses have an outer diameter of several mm (typically, about 4 mm).
In order to achieve further miniaturization of recording devices using for example optical disks, the object lens itself has to be made even smaller. However, since aspherical lenses are press-formed using a die, it is very difficult to make miniature aspherical lenses with a diameter of 1 mm or less (see "Bishoukougaku Handobukku", p. 6, Optical Society of Japan, 1995).
Rod lenses having a refractive index distribution in the radial direction can be used as optical imaging systems with an outer diameter of 1 mm or less. The refractive index distribution for this kind of rod lenses can be expressed, for example, as EQU n(r).sup.2 =n.sub.0.sup.2 .multidot.{1-(g.multidot.r).sup.2 +h.sub.4 (g.multidot.r).sup.4 +h.sub.6 (g.multidot.r).sup.6 +h.sub.8 (g.multidot.r).sup.8 +. . . } (Eq. 1)
wherein r is a distance in a radial direction from the optical axis of the rod lens; n(r) is the refractive index at a distance r in the radial direction from the optical axis of the rod lens; n.sub.0 is a refractive index on the optical axis of the rod lens (central refractive index); and g, h.sub.4, h.sub.6, h.sub.8, etc. are refractive index distribution coefficients.
Such a graded index lens can be manufactured, for example, from a rod-shaped glass by ion exchange (see for example "Hikarigijutsu Yougo Jiten" 2.sup.nd Ed., by Shuuji KOYANAGI, Optronics, Tokyo 1998, p. 12). Using ion exchange, a miniature lens with an outer diameter of not more than 1 mm can be manufactured at low cost. Furthermore, since the material itself has a positive refractive power, simple rod lenses having planar surfaces on both ends can be used as object lenses.
The refractive power of a rod lens having the refractive index distribution of Eq. 1 can be expressed by n.sub.0 .multidot.g.multidot.r.sub.0. The larger this value, the larger is the NA of the object lens. However, if a rod lens having a refractive index distribution in the radial direction is produced by ion exchange, there is a limit to the refractive index difference that can be attained, and a realizable range for n.sub.0 .multidot.g.multidot.r.sub.0 is n.sub.0 .multidot.g.multidot.r.sub.0 .ltoreq.0.71.
However, to increase the recording density of recording media even further, there is a need for an object lens system with an even larger NA. Moreover, to achieve both a smaller device and faster access speeds, the number of parts has to be kept to a minimum, while making the object lens system smaller and lighter.